1. Field of the Invention
This invention relates to a method and apparatus for producing and depositing monomolecular layers on a substrate. More specifically, this invention is concerned with a practical method for continuously producing and depositing monomolecular layers onto substrates.
2. Prior Art
Monomolecular layers of organic compounds find a large number of applications, especially in the field of electronics. For example, monomolecular layers are used in forming metal-insulator-metal structures in which the thickness of the dielectric insulator layer must be controlled with a particularly high degree of accuracy.
One known method for producing and depositing monomolecular layers of amphiphilic molecules has been described by Langmuir (J. Am. Chem. Soc., Vol. 57 (1935) pp 1007-1010). Amphiphilic molecules are those having a hydrophilic end and a hydrophobic end, and line up side-by-side in a particular direction.
The Langmuir method consists of forming a monomolecular layer at the surface of a tank filled with a liquid such as water. In accordance with this method, a solution of amphiphilic molecules dissolved in a solvent which is not miscible with the liquid of the tank is spread onto the liquid surface, whereupon the solvent evaporates so that a loosely packed monomolecular layer is formed on the liquid surface. A transition of the monomolecular layer thus formed from a state of gas or liquid to a solid state is then achieved by reduction of the surface area of the layer by compressing it to a predetermined surface pressure. The resulting monomolecular layer is deposited onto the surface of a substrate by passing the substrate through the compressed layer while maintaining the layer at the predetermined surface pressure during the period of deposition.
The standard method adopted for compressing the monomolecular layer consists of using a leak-tight moving barrier which moves over the liquid surface of the tank. By displacing the barrier, the area of the monomolecular layer which had previously been formed at the liquid surface of the tank is reduced to bring the layer to the desired surface pressure. When this value is attained, the deposition operation can begin. The surface pressure of the layer is maintained at this desired surface pressure value by displacing the moving barrier so as to provide progressive compensation for the space left by the molecules which have been deposited on the substrate. On completion of the deposition operation, the moving barrier is brought back to its original position, the surface of the liquid is cleaned of the residual material and then the entire surface of the liquid is again available for the introduction of a fresh quantity of amphiphilic molecule solution.
This standard method does not readily permit continuous formation and deposition of monomolecular layers onto substrates since it is necessary to periodically replenish the bath and to compress the layer. In addition, the continuity of the compressed layer is not readily reproducible. Further, the moving barrier cause stress in some areas of the monomolecular layer, often damaging the layer.
One method for continuously producing and depositing monomolecular layers of amphiphilic molecules on solid substrates is described in U.S. Pat. Nos. 4,093,757 and 4,224,896. Specifically, the liquid surface contained in a water tank is divided into two adjacent compartments that are separated by a horizontal rotatable cylinder which is partly immersed in the liquid of the tank. A compressed monomolecular layer is formed in one of the compartments by transferring an uncompressed layer from one compartment to the adjacent compartment by imparting rotational motion to the cylinder. The rotating cylinder applies pressure to the uncompressed layer to thereby compress it to a predetermined surface pressure. The continuous production and deposition of monomolecular layer on substrates is performed by continuously introducing a solution of amphiphilic molecules into one compartment, continuously transferring the molecules from that compartment to an adjacent compartment by imparting rotational motion to the cylinder, and continuously dipping a substrate into the adjacent compartment.
Although the method described above may work conceptually, it has several disadvantages which render the method impractical for commercial applications. Specifically, the construction and material of the tank and of the cylinder that separates adjacent tank compartments, are disclosed to be highly dependent upon the desired application. In addition, frequent cleaning is required because the cylinder is coated with amphiphilic molecules. Still further, the process uses moving items (e.g., the cylinder) in contact with the monomolecular layer which cause a breakdown of the monomolecular layer as these items tend to cause stress in the liquid surface thereby ruining the continuity of the layer. The system also has sealing problems, specifically around the ends of the cylinder, thus making it difficult to achieve and maintain a predetermined monomolecular layer surface pressure.